1. Field of the Invention
The present invention relates to a sheet punching apparatus for punching a sheet and a control method thereof.
2. Description of the Related Art
The sheet punching apparatus is classified into a press punch type for punching conveyed recording sheets one sheet by one sheet while temporarily stopping sheet conveyance and a rotary type for punching recording sheets without stopping sheet conveyance. The press punch type is generally more accurate in punched hole position and smaller in variation in punched hole position than in the rotary type.
To make the apparatus compact in size and reduced in cost, a punch drive motor comprised of a DC motor is used in the sheet punching apparatus. However, the DC motor is lower in stop accuracy as compared to a pulse motor and hence produces a large variation in stop position from a target stop position.
In order to suppress a variation in stop position, a punching apparatus described in U.S. Pat. No. 7,172,185 is configured to start, immediately after start of a punch motor, a time measurement in which pulses are counted by an encoder attached to the punch motor, and applies brake to the motor upon lapse of a predetermined time from the start of the motor.
The punching apparatus described in the U.S. patent causes the punch motor to restart so as to bring a punch blade close to a desired position, if a punch blade position detected at the time of or prior to a motor stop deviates from the desired position, thereby improving the accuracy of motor stop position.
Although the punching apparatus described in the U.S. patent is suitable for correction of a variation (individual difference) of the apparatus (punch motor), operation of the apparatus is liable to vary even if the apparatus operates at the same load. Thus, even when a timing at which brake is started to be applied to the punch motor is kept the same, the punch motor (a movable part of the apparatus) does not always stop at the same position, and therefore the accuracy of movement distance of the movable part is not high. FIG. 16 is a timing chart of a punch motor stop operation that entails a variation in motor stop position due to repetitive operations.
With the punching apparatus described in the U.S. patent that restarts the punch motor (DC brush motor) to correct a deviation of the stop position of the punch blade, the motor is repeatedly started and stopped, and therefore the service life of the motor is shortened, posing a problem that component parts of the apparatus must frequently be replaced and the maintenance cost of the apparatus increases.